Future diesel engines that exploit a novel state of matter in which heated and compressed diesel fuel behaves both like a liquid and a gas could release as much as 80% fewer exhaust emissions while running 10% more efficiently, according to chemists at Syracuse University.
The researchers have shown in lab tests that raising the temperature of diesel beyond its thermodynamic critical point—“hot enough that it does not become a liquid even at high pressures”—enables a supercritical (SC) fuel charge to mix with air much faster than standard diesel in conventional compression-ignition engines, said team leader George Anitescu.
Although diesel engines tend to be more efficient than gasoline power plants, the combustion of diesel oil—a viscous, low-volatility liquid—is usually incomplete, so diesels tend to emit sooty carbon particles. And because diesel fuel burns at higher temperatures than gasoline, more nitrogen oxides evolve. Powertrain engineers have labored for decades to overcome these two problems.
“The nice thing about SC fluids is their intermediate nature, which is enticing from the viewpoint of injecting fuel into a combustion chamber,” Anitescu said. Heating diesel fuel so that it goes supercritical markedly alters its physical and chemical properties. SC diesel, for example, exhibits “an order of magnitude less viscosity than regular diesel, so it passes through an injector nozzle faster,” he explained. “And because of its high molecular diffusivity and lack of surface tension, it combines with air and various diluents such as carbon dioxide or exhaust gas very rapidly.”
Preheating before injection could overcome long-standing timing issues in diesel combustion processes, whereby fuel droplets injected even at 2000 bar (29,007 psi) tend to adhere to the hot cylinder walls where they “splash-boil just like water droplets dancing on a hot plate before vaporizing,” noted the chemist. He came up with the idea three years ago while working on topics such as using SC water to oxidize PCBs for pollution remediation.
In a practical supercritical injection system, heat from the engine's exhaust could be used to raise the fuel's temperature to 400° to 450ºC at a pressure of about 600 bar (8702 psi). Anitescu expected the high pressures to cause few problems, but building an injector that can withstand such high temperatures will require a new design and heat-resistant materials.
The other problem, he continued, stems from the fact that “if you just heat diesel, it cokes and leaves a sticky carbon black deposit on the surfaces that can cause failures. That means you need to add a diluent that mixes in among the fuel molecules to slow the thermal decomposition reaction.” One of the best of these anti-coking agents is carbon dioxide; another is recirculated engine exhaust gas (EG).
Anitescu’s team has so far tested their concept using benchtop laboratory apparatus that includes a temperature-resistant sapphire viewing window through which the researchers can observe and film at high speed the reactions between the heated fuel and diluents. “We tried different temperatures, pressures, and flow rates,” he noted.
At a high temperature threshold, Anitescu's team observed a homogeneous phase result, which encouraged them since that’s what they believe is needed in the combustion chamber. "Although we still need to quantify the numbers, it clearly mixed much faster than liquid diesel,” he said.
The Syracuse researcher reported that his group is building a new manual injector as well as a larger view cell to analyze sprays and the mixing of fuel and diluents under various conditions. In the meantime, an engineering partner firm is developing a high-temperature injector and associated engine systems that could be ready by year’s end.
In an engine, it seems best that the fuel be heated to just below the critical point, Anitescu observed. He explained that this is appealing because the injector works better with a liquid. “You’d pump the preheated fuel into a small chamber that’s supplied with a stream of anti-coking EGR, and very quickly you’d have a liquid solution that’s about 10% EG.”
A common-rail injection system would deliver the near-SC fuel-diluent mixture to the cylinders. Keeping the charge in liquid form would also help control ignition timing and provide better injection penetration, he noted. And by precisely metering out the fuel injection, combustion could, for example, occur under efficient stoichiometric conditions.
The patented concept could eventually yield a variety of benefits, according to the researchers. They include near-complete combustion of diesel fuel and recovery of up to 50% of the exhaust heat; potential elimination of around 80% of criteria pollutants and a significant reduction in released heat; possible engine downsizing by using smaller cylinder displacement to produce the same torque and power; minimizing the parasitic pumping of large volumes of excess air; and the elimination of exhaust aftertreatment systems.
The new technology could also impact efforts to tame homogeneous charge compression ignition (HCCI), which boosts efficiency by injecting into the cylinder a premixed vapor of diesel and air under high pressure. But HCCI occurs spontaneously and sporadically over an ill-defined period, so timing control is difficult. In addition, engine knocking can result from ignition of isolated pockets of unburned fuel and air.
Near-SC diesel injection produces very small vapor-like droplets with overall densities equivalent to those of liquids, Anitescu said. “We expect to be able to operate somewhere between HCCI and conventional diesel modes,” he added.